Upon its completion in October 2001, the $123 million Midfield Terminal Parking Garage just beginning construction for Northwest Airlines at Detroit Metropolitan Airport will be the nation's largest to be built in one phase, according to officials. When the design was bid out with a choice of structural systems, the precast concrete option was found to offer savings of more than $18 million for the gigantic project.

The 10-level, 11,600-car garage will feature 89 acres of floor space in all, with 80 acres supported. To complete the project on time, five precasters operating seven plants will fabricate the needed components. The project is being built by AWP, a joint venture of J.S. Alberici Construction Co. in St. Louis and Livonia, Mich; Walsh Construction Co. in Chicago and Detroit; and the lead precaster, PBM Concrete Inc. in Rochelle, Ill. The design team includes Walker Parking Consultants in Kalamazoo, Mich.; HNTB Engineers in Detroit; and Sims Varner Architects also in Detroit.

Project delivery is being handled as a modified design-build contract, explains Tony Chrest, project manager for Walker. The company's designers produced a complete set of drawings that described the building in detail and showed the structural design in three systems: cast-in-place; post-tensioned precast concrete; and a composite structure with precast beams and columns and cast-in-place floor slabs. It was left to the winning contractor to complete the design work, including completion of structural design, a lighting system and a drainage system.

The project will require 5,000 pretopped double-tee girders, each 10 ft. wide by 60 ft. long. Shelby Precast Concrete Co. in Shelby Township, Mich., is delivering most of the double tees. Besides PBM and Shelby, the other precasters and plants are Nucon Schokbeton Inc. in Kalamazoo, Mich.; Hollowcore Inc., with plants in Detroit and Windsor, Ontario; and Prestress Services Inc., with plants in Decatur, Ind., and Lexington, Ky.

The parking structure is being built adjacent to a new terminal building for Northwest Airlines. Challenges on the project include limited access to the site, says Chrest. "Everything has to be brought into the site at one narrow end," he notes. "We've got an extremely tight site that requires working in the middle of the airfield."

Building the huge garage in prestressed has several advantages, explains Mark Morrow, precast coordinator for the AWP joint venture:

- Construction capacity - Precast pieces can be made at multiple plants, and there will be less concern over the availability of tradesmen.

- The quality of the concrete is better, as the Precast/Prestressed Concrete Institute certifies all seven plants. Enhanced durability of the concrete, another bonus, is a requirement since specifications call for 40-year service life.

- Less impact from the weather - The pretopped double-tee girders minimize field-placed concrete so that erection and construction can continue all year.

- Speed of assembly - The construction schedule is telescoped because the precast components are manufactured as foundations are constructed. Using precast will take approximately two months less than other methods, Project Manager Tony Chrest estimates.